Reducing speckle pattern in display images

ABSTRACT

A display system includes a coherent light source that can emit a coherent light beam, a de-speckling device configured to distort a wavefront of the coherent light beam to produce a distorted coherent light beam, and a two-dimensional array of light modulators that can selectively modulate the distorted coherent light beam to select a plurality of pixels for display.

BACKGROUND

The present disclosure relates to the displaying images employing acoherent light source.

Coherent light sources such as laser devices are commonly used indisplay systems. When a rough surface is illuminated by a coherent laserbeam, the rough surface usually exhibits a salt-and-pepper speckledappearance. The seemingly random pattern is referred to as a “specklepattern.” The speckle pattern is a random intensity pattern produced bythe interference of the laser's coherent wavefronts reflected off therough surface. When a laser is used as a light source in a displaysystem, a speckle pattern often superimposes on the display imageproduced by the display system. The speckle pattern can be ratherdistracting to the viewer and can degrade the quality of the displayimage.

SUMMARY

In one general aspect, the present invention relates to a display systemincluding a coherent light source that can emit a coherent light beam, ade-speckling device configured to distort a wavefront of the coherentlight beam to produce a distorted coherent light beam, and atwo-dimensional array of light modulators that can selectively modulatethe distorted coherent light beam to form a display image comprising aplurality of pixels on a display surface.

In another general aspect, the present invention relates to a method fordisplaying an image. The method includes receiving a coherent light beamfrom a coherent light source, distorting a wavefront of the coherentlight beam to produce a distorted coherent light beam; and selectivelymodulating the distorted coherent light beam by a two-dimensional arrayof light modulators to form a display image comprising a plurality ofpixels on the screen surface.

Implementations of the system may include one or more of the followingfeatures: The de-speckling device can includes an optical mediumpositioned in the path of the coherent light beam between the coherentlight source and the two-dimensional array of light modulators; and anactuator configured to produce an acoustic wave in the optical medium todistort the wavefront of the coherent light beam. The actuator caninclude a piezoelectric material mechanically coupled to the opticalmedium and a controller that can produce an alternating electric fieldin the piezoelectric material to produce the acoustic wave in theoptical medium. The de-speckling device can further include one or moreelectrodes configured to receive electric voltage signals from thecontroller. The optical medium can include a dichroic mirror, a mirror,or an optical diffuser. The optical medium can be at least partiallytransparent to the coherent light beam. The optical medium can includeglass or a transparent plastic material. The de-speckling device caninclude a heater configured to produce turbulent air currents in thepath of the coherent light beam by heating air, wherein the turbulentair currents distort the wavefront of the coherent light beam. Theheater can include a resistive element configured to be heated aboveambient temperature in response to a voltage applied across theresistive element. The heater receives heat generated by the coherentlight source or the two-dimensional array of light modulators. Thede-speckling device can distort the wavefront of the coherent light beamby 0.1 to 100 microns while keeping the two-dimensional array of lightmodulators fully illuminated by the distorted coherent light beam. Thede-speckling device can distort the coherent light beam at a frequencyhigher than 60 Hz. The de-speckling device can distort the coherentlight beam at a frequency higher than 1 KHz. The two-dimensional arrayof light modulators each can include a micro mirror configured toselectively modulate the distorted coherent light beam to form a displaypixel on the screen surface or away from the screen surface. The micromirror can include a tiltable mirror plate having a reflective surface,wherein the tiltable mirror plate is configured to tilt to an “on”position by an electrostatic force to reflect distorted coherent lightbeam to form the display pixel on the screen surface.

Various implementations of the methods and devices described herein mayinclude one or more of the following advantages. The disclosed systemsand methods can improve the viewing quality of display images producedby display systems using coherent light sources. Furthermore, disclosedsystems and methods are generally applicable to different types ofspatial light modulators.

Although the invention has been particularly shown and described withreference to multiple embodiments, it will be understood by personsskilled in the relevant art that various changes in form and details canbe made therein without departing from the spirit and scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings, which are incorporated in and form a part of thespecification, illustrate embodiments of the present invention and,together with the description, serve to explain the principles, devicesand methods described herein.

FIG. 1 is a schematic diagram of an exemplified display system includinga coherent light source and a de-speckling device.

FIG. 2 is a schematic top view of a spatial light modulator compatiblewith the display system of FIG. 1.

FIG. 3 is a cross-sectional view of an exemplary micro mirror in thespatial light modulator of FIG. 2.

FIGS. 4A and 4B are schematic diagrams of an exemplified de-specklingdevice compatible with the display system of FIG. 1.

FIGS. 5A and 5B are schematic diagrams of another exemplifiedde-speckling device compatible with the display system of FIG. 1.

FIG. 6 is a schematic diagram of another exemplified arrangement forreducing speckle in a display system.

DETAILED DESCRIPTION

Referring to FIG. 1, a display system 10 includes a spatial lightmodulator 20 on a support member 25, a total internal reflection (TIR)prism 50, an opaque aperture structure 70 having an opening 75, and aprojection system 60. The display system also includes one or morecoherent light sources such as red, green, and blue light sources 80 a,80 b and 80 c, diffusers 85 a, 85 b, 85 c, and dichroic mirrors 90 b and90 c. Examples of the red, green, and blue light sources 80 a, 80 b and80 c include solid-state semiconductor lasers, diode pumped lasers orion or gas laser devices. The red, green, and blue light sources 80 a,80 b and 80 c can sequentially emit coherent red, green, and blue lightbeams 330 a, 330 b, and 330 c to illuminate the spatial light modulator20. The sequentially emitted red, green, and blue colored light 330 a,330 b, and 330 c respectively pass through diffusers 85 a, 85 b, and 85c to form colored light 331 a, 331 b, and 331 c. The diffusers 85 a, 85b, and 85 c are configured to resize (e.g., expand) and shape thecross-sections of the light beams 330 a, 330 b, and 330 c to becompatible with the shape of the spatial light modulator 20. Forexample, for a rectangular shaped spatial light modulator 20, thecolored light 330 a, 330 b, and 330 c can be shaped by the diffusers 85a, 85 b, 85 c to have rectangular cross-sections, which can becompatible with the spatial light modulator 20.

The colored lights 331 b and 331 c are respectively reflected bydichroic mirrors 90 b and 90 c. The dichroic mirrors 90 b and 90 c alsofunction as beam combiners, which merge colored lights 331 a, 331 b and331 c into color light 332 along a common optical path. The coloredlight 331 a passes through the dichroic mirror 90 b, and then travelsalong the same optical path as the colored light 331 b, 331 c after thecolored light 331 b, 331 c are redirected. The color light 332represents one of the colored lights 331 a, 331 b, or 331 c at each timebecause the colored lights 330 a, 330 b, or 330 c are sequentiallyemitted. The color light 332 subsequently passes a de-speckling device200. In the present application, the term “de-speckling device” refersto a device that can alter an incident coherent light beam in such a waythat the speckle pattern can be reduced or eliminated in the displayimage. In some embodiments, the de-speckling device 200 can slightlydistort the wavefront of the colored light 332 to produce a coloredlight 333 that is displaced (i.e., wobulated) randomly over time insmall amplitudes in two dimensions.

The colored light 333 is reflected by the TIR prism 50 to form coloredincident light 330, which illuminates micro mirrors in the spatial lightmodulator 20. The colored incident light 330 is selectively reflected bythe spatial light modulator 20 to form a reflected light beam 340 whichpasses through the TIR prism 50 and the opening 75 in the aperturestructure 70. When the red colored light 330 a is emitted, the coloredlight 331 a and the colored incident light 330 are red. The reflectedlight beam 340 is projected by the projection system 60 to form a redimage on a screen surface 40. Similarly, when the green and blue coloredlights 330 b, 330 c are respectively emitted, the colored incident light330 is respectively green and blue. The reflected light beam 340 isprojected by the projection system 60 to respectively form a green and ablue image on a screen surface 40. The red, green, and blue displayimages (i.e., color planes) can appear to be superimposed in the eyes ofa viewer to have the visual effect of a color display image. As shown inFIG. 2, the spatial light modulator 20 includes an array 110 of lightmodulators 150. The color display image includes a display pixel 30(FIG. 1) that is produced by one of the light modulators 150 in thearray 110.

Referring to FIG. 3, an exemplified light modulator 150 includes amirror plate 202 that includes a flat reflective upper layer 203 a, amiddle layer 203 b that provides mechanical strength to the mirror plate202, and a bottom layer 203 c. The upper layer 203 a is formed of areflective material such as aluminum, silver, or gold. The upper layer203 a can have a thickness in the range of between about 200 and 1000angstroms, such as about 600 angstroms. The middle layer 203 b can beformed by a rigid material such as amorphous silicon, a metal or analloy, typically about 2000 to 5000 angstroms in thickness. The bottomlayer 203 c can be made of an electrically conductive material thatallows the electric potential of the bottom layer 203 c to be controlledto be different from the electric potentials of step or comb electrodes221 a or 221 b. The bottom layer 203 c can be made of titanium, atitanium alloy, or other metallic alloys or doped semiconductors. Thebottom layer 203 c can have a thickness in the range of about 200 to1000 angstroms.

A hinge 206 is connected with the bottom layer 203 c (the connectionsare out of plane of view and are thus not shown in FIG. 3). The hinge206 is supported by a hinge post 205 that is rigidly connected to thesubstrate 120. The mirror plate 202 can include two hinges 206 connectedto the bottom layer 203 c of the mirror plate 202. The two hinges 206define a rotational axis about which the mirror plate 202 is able totilt. The hinges 206 extend into cavities in the lower portion of mirrorplate 202. For ease of manufacturing, the hinge 206 can be fabricated aspart of the bottom layer 203 c.

Step electrodes 221 a and 221 b, landing tips 222 a and 222 b, and asupport frame 208 are also fabricated over the substrate 120. Theheights of the step electrodes 221 a and 221 b can be in the range frombetween about 0.05 microns and 3 microns. The step electrode 221 a iselectrically connected to an electrode 281 with a voltage V_(d) that isexternally controlled. Similarly, the step electrode 221 b iselectrically connected with an electrode 282 with a voltage Va that canalso be externally controlled. The electric potential of the bottomlayer 203 c of the mirror plate 202 can be controlled by an electrode283 at potential V_(b). Electric pulses applied to the electrodes 281,282, and 283 can create electric potential differences between thebottom layer 203 c in the mirror plate 202 and the step electrodes 221 aor 221 b, which produces electrostatic forces on the mirror plate 202.An imbalance between the electrostatic forces on the two sides of themirror plate 202 can cause the mirror plate 202 to tilt from oneorientation to another. The landing springs 222 a and 222 b areconfigured to stop the mirror plate's 202 tilt movement at a preciseangle. The landing springs 222 a and 222 b are able to store elasticstrain energy when they are deformed by electrostatic forces. Theelastic strain energy can be converted to kinetic energy to push awaythe mirror plate 202 when the electrostatic forces are removed. Thepush-back on the mirror plate 202 can help separate the mirror plate 202and the landing springs 222 a and 222 b. In some embodiments, the middlelayer 203 b includes cavities 223 a and 223 b, which respectively formsmembranes 224a and 224b above the landing springs 222 a and 222 b.Similar to the landing springs 222 a and 222 b, the membranes 224 a or224 b can also store elastic energies when the mirror plate 202 istilted to contact the landing spring 222 a or 222 b under electrostaticforces. The elastic energy stored in the membranes 224 a or 224 b canseparate the mirror plate 202 from the landing tip 222 a or 222 b whenthe electrostatic forces are removed. In some embodiments, the landingsprings 222 a and 222 b are electrically connected to the hinge post 205and to the electrodes 283 so that there is no potential differencebetween the landing springs and bottom layer 203 c of the mirror plate202 when these members come into mechanical contact. Alternatively, themicro mirror can be formed without landing springs 222 a and 222 b. Suchdevices without landing springs can include a cantilever spring, bridgespring or hinge layer connected to the mirror with stitches.

The electrical pads 281, 282, 283 are electrically connected toelectrical pads 112 adjacent to the array 110 of light modulator 150. Inoperation, the electrical pads 112 can receive control electricalsignals from a control circuit. The control electrical signals canindividually address light modulator 150 and produce electrostaticforces on the mirror plate 202 to tilt the mirror plate 202 to an “on”position and an “off” position. When the mirror plate 202 is at an “on”position, as shown in FIG. 3, the color incident light 330 forms anincident angle θ_(In) relative to the normal direction of the mirrorplate 202. The reflected light beam 340 forms a reflective angle θ_(Ref)of equal value relative to the normal of the mirror plate 202. Thedirections of the color incident light 330 and the reflected light beam340, and the orientation of the mirror plate at the “on” position can bearranged so that the reflected light beam 340 is substantially verticalto the top surface of the substrate 120. When the mirror plate 202 istilted to reflect light in the “off” position, the light is directedaway from a screen surface, such as to an absorbing surface orsufficiently far enough away from the light directed in the “on”position to cause little to no interference. Thus, at any one moment intime, neighboring pixels on the screen surface can be “on” or “off”,depending on the orientation of mirror plates in the array 110.

Referring now to FIG. 1, the colored light 332 is continuous andspatially non-discrete. The coherence pattern in the colored light 332is defined by the red, green, and blue colored light 330 a, 330 b, and330 c respectively emitted by the red, green, and blue light sources 80a, 80 b and 80 c. The reflected light beam 340 has been spatiallymodulated by the array 110 of the light modulators 150 (FIG. 2). In theabsence of the de-speckling device 200, the colored light 332 wouldtravel along a straight and fixed optical path to the TIR prism 50. Thecoherence in the reflected light beam 340 can create undesirable specklepatterns (i.e., a “salt and pepper pattern”) in the display image on thescreen surface 40.

The de-speckling device 200 is introduced to dynamically disturb thecoherence pattern in the colored light 332 in order to reduce thespeckle pattern in the display image. Referring to FIGS. 1, 4A and 4B,the de-speckling device 200 includes a transparent media 410 having afront surface 411 and a back surface 412, an actuator 420 in contactwith the transparent media 410, and a controller 430. The transparentmedia 410 can be formed by glass or a transparent plastic material. Theactuator 420 can be a piezoelectric material in contact with a pair ofelectrodes (not shown). Without actuation by the actuator 420, as shownin FIG. 4A, the front surface 411 and the back surface 412 are stableand remain flat. The colored light 332 passes through and exits thetransparent media 410 to form the colored light 333. The colored light333 is moving dynamically over small distances, but has substantiallythe same propagation direction and beam spread of the colored light 332.

The controller 430 is configured to apply voltages at the appropriateamplitudes and frequencies to actuate the actuator 420. For example, thecontroller 430 can create an alternating electric field in thepiezoelectric material of actuator 420 to induce vibrations in thedirection 421. The frequency of the vibration is controlled to be higherthan the video refresh rate of 60 Hz. For example the vibrationfrequency can be in a range between 1 kHz and 10 MHz. The vibrationfrequency can be twenty to one hundred thousand times the video refreshrate.

Referring to FIG. 4B, the vibration of the actuator 420 can createsurface acoustic waves 415, 416 respectively at the front surface 411and/or the back surface 412 of the transparent media 410. The surfacewaves 415, 416 can distort the wavefront of the colored light 332 at thefront surface 411 and/or the back surface 412, and undulate the coloredlight 333 in the direction 99. The beam of the colored light 333 is alsosomewhat more divergent over a cycle of surface vibration due to thediffractions by the rippled surface waves 415, 416. The colored light333 is still continuous and spatially non-discrete; only its wavefrontis distorted.

Referring to FIGS. 1-4, the wavefront of the colored incident light 330is dynamically distorted over time. The wavefront of the coloredincident light 330 can be dynamically displaced by between 0.1 to 100microns, depending on the size of the picture element in the projectedimage. The coherent light beam is broad enough to keep the array 110 oflight modulator 150 (FIG. 2) fully illuminated during the displacementsand the distortion of the wavefront. The magnitude of the displacementsof the colored incident light 330 can be controlled by the voltageapplied to the actuator 420 by the controller 430. The magnitude of thedisplacements of the colored incident light 330 on the spatial lightmodulator 20 can be controlled to be larger than the wavelength of thecolored incident light 330. For example, the displacements can rangebetween 0.1 and 100 microns, or between 1 and 10 microns. As the coloredincident light 330 moves in a vibration cycle, different portions of thecolored incident light 330 come to illuminate a particular mirror plate202 in a light modulator 150. Since the different portions of thecoherent color incident light 330 have different coherence phases, thespeckle pattern produced by the reflected light beam 340 on the screensurface 40 shifts in accordance to the movements in the colored light333. Since the movements of the colored light 333 and the coloredincident light 330 have a frequency higher than the video frame rate,the shifted speckle patterns on the screen surface 40 are averaged outwithin the period of a video frame. The visual effects of the “salt andpepper” speckle patterns can thus be reduced or eliminated.

In some embodiments, referring to FIGS. 5A and 5B, the de-specklingdevice 200 includes a heater 520 positioned slightly below the path ofthe colored light 332. A controller 530 can control the temperature ofthe heater 520. In one implementation, the heater 520 can include athermal resistor that can be heated by an electric current produced by avoltage applied by the controller 530. When the heater 520 is heated toa certain temperature above ambient temperature, turbulent air currents515 can be produced in the air above the heater 520 and in the path ofthe colored light 332. For example, the heater temperature can beelevated to 20° C. to 80° C. higher than the ambient temperature tocreate turbulent air currents in the path of the colored light 332. Theturbulent air currents 515 are hotter than the surrounding air and thushave a slightly lower and spatially variable refractive index. Theturbulent air can distort the wavefront and scramble the coherent phasesof the colored light 332. The wavefront of the colored incident light330 (FIG. 1) is thus distorted and can move across the spatial lightmodulator 20 (FIG. 1). As described above, the displacements of thecolored incident light 330 on the spatial light modulator 20 can becontrolled to be larger than the wavelength of the colored light 333,and at a frequency higher than the refresh rate of the video frames.

In some embodiments, turbulent air currents can be generated by heatgenerated by one or more components in the display system. Heat isusually generated by light sources (e.g., 80 a-80 c in FIG. 1) such aslasers, which are conventionally cooled by forcing air over their heatsinks. Another heat source is the spatial light modulator 20 (FIG. 1).It absorbs part of incident light and has to be cooled in higher outputillumination applications. Instead of being actively heated using anexternal power, the heater 520 (shown in FIGS. 5A and 5B) can be heatedby thermal conduction from a heat-generating light source or the spatiallight modulator. The colored light 332 can pass through the hot,turbulent air above the heater 520 to reduce laser speckle in thedisplay image. In this arrangement, the heater 520 also serves as acooling device for the light source.

In some embodiments, a de-speckling device can also perform otherfunctions in a display system. Referring to FIG. 6, where the numbersthat are the same as in FIG. 1 represent the same elements, actuators620 a, 620 b are respectively attached to the dichroic mirrors 90 b, 90c. The actuators 620 a, 620 b are controlled by a controller 630.Similar to the description above of the embodiment shown in FIGS. 4A and4B, the actuators 620 a, 620 b can be controlled by the controller 630to create surface acoustic waves in the dichroic mirrors 90 b, 90 c,which can distort the wavefronts of the colored light 331 a, 331 b, and331 c and undulate the colored light 331 a, 331 b, and 331 c in thedirection 99. As a result, the speckle patterns in the display image canbe reduced or eliminated. Surface acoustic waves can also be generatedin other reflective or transmissive optical components such asdiffusers, lenses, and mirrors to reduce speckling in the display image.

It is understood that the disclosed systems and methods are compatiblewith other configurations of spatial light modulators such as otherreflective devices, i.e., LCoS devices, or transmissive liquid crystaldevices. Moreover, the time-varying distortions of the wavefront of thecoherent incident light can be achieved by different mechanisms andusing different optical components. The de-speckling device can bepositioned in the light path before the incident light impinges thespatial light modulator or after light has been reflected by the spatiallight modulator.

What is claimed is:
 1. A display system comprising: a coherent lightsource configured to emit a coherent light beam having a path and awavefront; a de-speckling device positioned in the path of the coherentlight beam between the coherent light source and a two-dimensional arrayof light modulators, the de-speckling device configured to distort thewavefront of the coherent light beam to produce a distorted coherentlight beam; and a two-dimensional array of light modulators configuredto selectively modulate the distorted coherent light beam to form adisplay image comprising a plurality of pixels, wherein the de-specklingdevice includes: an optical medium; and an actuator configured toproduce an acoustic wave in the optical medium that generates atranslational, rotational or tilting motion of the wavefront of thecoherent light beam.
 2. The display system of claim 1, wherein theactuator comprises: a piezoelectric material mechanically coupled to theoptical medium; and a controller configured to produce an alternatingelectric field in the piezoelectric material to produce the acousticwave in the optical medium.
 3. The display system of claim 2, whereinthe de-speckling device further comprises one or more electrodesconfigured to receive electric voltage signals from the controller. 4.The display system of claim 1, wherein the optical medium includes adichroic mirror, a mirror, or an optical diffuser.
 5. The display systemof claim 1, wherein the optical medium is at least partially transparentto the coherent light beam.
 6. The display system of claim 5, whereinthe optical medium comprises glass or a transparent plastic material. 7.The display system of claim 1, wherein the de-speckling device comprisesa heater configured to produce turbulent air currents in the path of thecoherent light beam by heating air, wherein the turbulent air currentsdistort the wavefront of the coherent light beam.
 8. The display systemof claim 7, wherein the heater includes a resistive element configuredto be heated above ambient temperature in response to a voltage appliedacross the resistive element.
 9. The display system of claim 7, whereinthe heat of the heater is generated by the coherent light source or thetwo-dimensional array of light modulators.
 10. The display system ofclaim 1, wherein the de-speckling device is configured to distort thewavefront of the coherent light beam by 0.1 to 100 microns while keepingthe two-dimensional array of light modulators fully illuminated by thedistorted coherent light beam.
 11. The display system of claim 1,wherein the de-speckling device is configured to distort the wavefrontof the coherent light beam at a frequency higher than 60 Hz.
 12. Thedisplay system of claim 1, wherein the de-speckling device is configuredto distort the wavefront of the coherent light beam at a frequencyhigher than 1 KHz.
 13. The display system of claim 1, wherein thetwo-dimensional array of light modulators comprises a micro mirrordevice including a plurality of micro mirrors configured to selectivelymodulate the distorted coherent light beam to select pixels for display.14. The display system of claim 13, wherein the micro mirrors comprise atiltable mirror plate having a reflective and configured to tilt to an“on” position when a pixel is to be selected.
 15. The display system ofclaim 1, wherein the de-speckling device is configured to distort thecoherent light beam at a frequency between about 1 KHz and about 10 MHz.